1. Field of the Invention
The present invention relates to an angular velocity sensor that measures angular velocity based on a Coriolis force. In particular, the present invention relates to an angular velocity sensor that drives a vibrator so as to vibrate in an in-plane direction of a plate surface of the vibrator and that measures angular velocities based on detected in-plane-direction or out-of-plane-direction vibration generated by a Coriolis force.
2. Description of the Related Art
First, examples of a configuration of a known angular velocity sensor are described. Following, an axis extending along a direction (thickness direction) perpendicular to a plate surface of an angular velocity sensor is assumed to be the z-axis of a Cartesian coordinate system. Meanwhile, two axes that extend along the plate surface and are orthogonal to each other are assumed to be the x-axis and the y-axis of the Cartesian coordinate system.
FIG. 4A is an xy-plane plan view of an angular velocity sensor 101 according to a first known example (see International Publication No. 2008/023566, for example).
The angular velocity sensor 101 includes a support part 102, four second arms 103A, 103B, 103C, and 103D, four weight parts 104, two first arms 105, two third arms 106, four fixing parts 107, a drive part 108, a detection part 109, and sensing parts 110 and 111. The support part 102 is disposed at the center of the xy-plane. An end of each of the second arms 103A to 103D is connected to the support part 102. Two of the second arms 103A to 103D extend from the support part in a direction away from the center so as to be parallel to each other in the direction of the y-axis, are bent near respective tips so as to extend parallel to the x-axis in respective directions away from the center, and are further bent so as to extend parallel to the y-axis in a direction toward the center; the other two of the second arms 103A to 103D extend from the support part in the opposite direction away from the center so as to be parallel to each other in the direction of the y-axis, are bent near respective tips so as to extend parallel to the x-axis in respective directions away from the center, and are further bent so as to extend parallel to the y-axis in the opposite direction toward the center. The four weight parts 104 are connected to the respective tips of the four second arms 103A to 103D. An end of each of the two first arms 105 is connected to the support part 102 and extends parallel to the x-axis. Each of the two third arms 106 extends parallel to the y-axis, and the center of the third arm 106 is connected to the other end of a corresponding one of the first arms 105. Two of the four fixing parts 107 are connected to the two respective ends of a corresponding one of the two third arms 106, whereas the other two of the four fixing parts 107 are connected to the two respective ends of the other one of the two third arms 106. The drive part 108 is provided to the second arm 103A and has a function of driving the four second arms 103A to 103D and the four weight parts 104 so that the four second arms 103A to 103D and the four weighting parts 104 vibrate along the x-axis. The detection part 109 is provided to the second arm 103B and has the function of detecting a drive vibration state. The sensing parts 110 and 111 are respectively provided to the second arms 103C and 103D and each have the function of sensing distortion of a corresponding one of the second arms 103C and 103D.
FIG. 4B is a perspective view of an angular velocity sensor 201 according to a second known example (see Japanese Unexamined Patent Application Publication No. 2011-158319, for example).
The angular velocity sensor 201 includes a base part 202, detection beams 203A to 203D, and a frame body 206. The base part 202 is located at the center of a plate surface of the angular velocity sensor 201. The detection beams 203A to 203D individually extend from the base part 202 in a cross shape. One end of each of the detection beams 203A to 203D is connected to the base part 202. The other end of each of the detection beams 203A to 203D is connected to the frame body 206. The frame body 206 has a substantially square shape in a plan view and includes corners 204A to 204D, which are located at the respective apexes of the substantial square, and drive beams 205A to 205D, which connect the corners 204A to 204D. Masses 207A to 207D are respectively attached to the drive beams 205A to 205D. Each of the masses 207A to 207D includes a pair of auxiliary masses, which are provided so that a corresponding one of the drive beams 205A to 205D is located therebetween. The pairs of auxiliary masses, which constitute the respective masses 207A to 207D, are connected to respective central portions of the drive beams 205A to 205D.
Piezoelectric drive elements 210 to 213 are provided on the respective surfaces of the drive beams 205A to 205D. Each of the piezoelectric drive elements 210 to 213 includes a pair of piezoelectric elements. The piezoelectric elements, which constitute each of the piezoelectric drive elements 210 to 213, are disposed parallel to each other in the extending direction of a corresponding one of the drive beams 205A to 205D. The piezoelectric drive elements 210 to 213 expand and contract upon application of a drive voltage. The drive beams 205A to 205D are respectively driven by the piezoelectric drive elements 210 to 213 and vibrate along the xy-plane so as to be alternately displaced in the direction toward the base part 202 and in the direction away from the base part 202. The drive vibrations of the drive beams 205A to 205D are excited in the same phase.
Piezoelectric detection elements 214 to 217 are provided on the respective surfaces of the detection beams 203A to 203D. Each of the piezoelectric detection elements 214 to 217 includes a pair of piezoelectric elements. The piezoelectric elements, which constitute each of the piezoelectric detection elements 214 to 217, are disposed parallel to each other in the extending direction of a corresponding one of the detection beams 203A to 203D. When an angular velocity acts on the angular velocity sensor 201, the detection beams 203A to 203D vibrate by the action of a Coriolis force. The piezoelectric detection elements 214 to 217 respectively detect vibrations of the detection beams 203A to 203D. More specifically, when an angular velocity in the direction around the z-axis acts on the angular velocity sensor 201 in a state where the drive beams 205A to 205D are being driven to vibrate, a Coriolis force acts in a direction perpendicular to the direction in which the angular velocity and the drive vibration direction is generated in the masses 207A to 207D. In other words, a Coriolis force is generated in the direction parallel to the direction in which each of the drive beams 205A to 205D extends in a stationary state. The masses 207A to 207D are displaced (detection vibrations) by the action of the Coriolis force. The detected vibrations of the masses 207A to 207D are respectively transmitted to the detection beams 203A to 203D via the drive beams 205A to 205D and the corners 204A to 204D in order to cause the detection beams 203A to 203D to vibrate. The vibrations of the detection beams 203A to 203D are detected by the piezoelectric detection elements 214 to 217.
The above-described angular velocity sensor 101 is capable of measuring the angular velocities around the y-axis and the z-axis but is incapable of measuring the angular velocity around the x-axis. For this reason, in order to measure the angular velocity around each of all of the x-axis, y-axis, and z-axis, an angular velocity sensor for measuring the angular velocity around the x-axis needs to be additionally provided. This causes a problem such as an increase in package size and an increase in cost. The angular velocity sensor 101 also has the following problems. When the angular velocity around the z-axis acts on the angular velocity sensor 101, the vibrations of the weights apply torque to the support part, and the detection vibrations escape from the vibrator instead of being kept within the vibrator. In this case, the beams are not efficiently deformed, consequently reducing the detection sensitivity of the angular velocity sensor 101. In addition, the above-mentioned detection vibrations may be caused by a stress or vibrations acting on an external structure, or the characteristics of the angular velocity sensor 101 may change due to changes in temperature or stress on the base, consequently reducing the detection accuracy of the angular velocity sensor 101.
Meanwhile the above-described angular velocity sensor 201 has the following problem. Specifically, when the angular velocity acts around the z-axis, the vibrations of the weights apply torque to the support part, and the detection vibrations escape from the vibrator instead of being kept within the vibrator. In this case, the beams are not efficiently deformed, consequently reducing the detection sensitivity and the detection accuracy of the angular velocity sensor 201.